1. Section 4: The Outer Planets
Preview
Key Ideas
The Outer Planets
Gas Giants
Jupiter
Saturn
Uranus
Neptune
Objects Beyond Neptune
Exoplanets
Light Year

2. Key Ideas
Identify the basic characteristics that make the outer planets different from terrestrial planets.
Compare the characteristics of the outer planets.
Explain why Pluto is now considered a dwarf planet.

3. The Outer Planets
The four planets farthest from the sun, Jupiter, Saturn, Uranus, and Neptune, are called the outer planets and are separated from the inner planets by a ring of debris called the asteroid belt.
gas giant a planet that has a deep massive atmosphere, such as Jupiter, Saturn, Uranus, and Neptune.
Usually found past the orbit of Neptune is Pluto. At one time, Pluto was considered the most distant planet. But in 2006, Pluto was reclassified as a dwarf planet.

4. Gas Giants
Gas giants are larger and more massive than terrestrial planets, but much less dense.
Unlike terrestrial planets, gas giants did not lose their original gases during formation.
Each gas giant has a thick atmosphere made mostly of hydrogen and helium gases.
Each planet probably has a core made of rock and metals.
All four gas giants have ring systems that are made of dust and icy debris that orbit the planets.

5. Jupiter
Jupiter is the fifth planet from the sun and is by far the largest planet in the solar system. Its mass is more than 300 times that of Earth and twice that of all the other planets combined.
The orbital period of Jupiter is almost 12 years. Jupiter rotates on its axis faster than any other planet—once every 9 h and 50 min.
Jupiter has at least 63 moons, 4 of which are the size of small planets. It also has several thin rings that are made up of millions of particles.

6. Jupiter, continued Jupiter’s Atmosphere
Hydrogen and helium make up 92% of Jupiter, so Jupiter’s composition is much like the sun.
Jupiter never became a star, like the sun, because it did not have enough mass to allow nuclear fusion to begin.
The orange, gray, blue, and white bands on Jupiter’s surface suggest the presence of organic molecules mixed with ammonia, methane, and water vapor.
Jupiter also has lightning storms and thunderstorms that are much larger than those on Earth.

7. Reading check, continued
Why didn’t Jupiter become a star?
When Jupiter formed, it did not have enough mass for nuclear fusion to begin.

8. Jupiter, continued Weather and Storms on Jupiter Jupiter’s Interior
Jupiter’s Great Red Spot is a hurricane-like storm that has been raging for at least several thousand years. Several other storms can be seen, although they are usually white.
The Galileo spacecraft measured wind speeds up to 540 km/h on Jupiter. Scientists have concluded that Jupiter’s internal heat affects the planet’s weather more than heat from the sun does.
Jupiter’s Interior
Jupiter’s large mass causes the temperature and pressure in Jupiter’s interior to be much greater than they are inside Earth.
With temperatures as high as 30,000 ºC, Jupiter’s interior is a sea of liquid, metallic hydrogen. Scientists think that Jupiter has a solid, rocky, iron core at its center.

9. Saturn
Saturn is the sixth planet from the sun, with an orbital period of 29.5 years. Saturn rotates on its axis every 10 h and 30 min.
Saturn is very cold and has an average cloud-top temperature of –176 °C.
Saturn has at least 60 moons. Its largest moon, Titan, is half the size of Earth.
Like Jupiter, Saturn is made almost entirely of hydrogen and helium, and has a rocky, iron core at its center. Saturn is the least dense planet in the solar system.

10. Saturn, continued Saturn’s Bands and Rings
Saturn is known for its rings, which are 2 times the planet’s diameter, and is the most complex and extensive system of rings.
Saturn’s rings are made of billions of dust and ice particles. Most of the debris probably came from comets or other bodies.
Like Jupiter, Saturn has bands of colored clouds that run parallel to its equator.
Saturn’s rapid rotation, paired with its low density, causes Saturn to bulge at its equator and to flatten at its poles.

11. Reading check, continued
How is Saturn similar to Jupiter?
Saturn and Jupiter are made almost entirely of hydrogen and helium and have rocky-iron cores, ring systems, many satellites, rapid rotational periods, and bands of colored clouds.

12. Uranus
Uranus is the seventh planet from the sun and the third-largest planet in the solar system. The orbital period of Uranus is almost 84 years.
Discovered by Sir William Herschel in 1781, Uranus is a difficult planet to study because it is nearly 3 billion kilometers from the sun.
The Hubble Space Telescope has taken images to show changes in Uranus’s atmosphere.
Uranus has at least 27 moons and at least 12 thin rings.
Uranus’s Rotation
Most planets rotate with their axes perpendicular to their orbital planes as they revolve around the sun. However, Uranus’s axis is almost parallel to the plane of its orbit and rotates once about every 17 h.

13. Uranus, continued Uranus’s Atmosphere
Like the other gas giants, Uranus has an atmosphere that contains mainly hydrogen and helium.
The blue-green color of Uranus indicates that the atmosphere also contains significant amounts of methane.
The average cloud-top temperature of Uranus is –214 °C. However, astronomers believe that the planet’s temperature is much higher below the clouds.
Scientists think that the center of Uranus, which has a temperature of about 7,000 ºC, has a core of rock and melted elements.

14. Neptune
Neptune is the eighth planet from the sun and is similar to Uranus in size and mass.
The orbital period of Neptune is nearly 164 years. Neptune rotates about every 16 h.
Neptune has at least 13 moons and six rings.
The Discovery of Neptune
Neptune’s existence was predicted before Neptune was actually discovered. Scientists suspected that the gravity of unknown planet was responsible for the variations in Uranus’s orbit.

15. Neptune, continued Neptune’s Atmosphere
Neptune’s atmosphere is made up mostly of hydrogen, helium, and methane.
Neptune has the solar system’s strongest winds, which exceed 1,000 km/h.
The Great Dark Spot on Neptune was a giant storm the size of Earth that appeared and disappeared on Neptune’s surface.
Neptune’s average cloud-top temperature is about –225 °C.

16. Objects Beyond Neptune
Pluto, now defined as a dwarf planet, orbits the sun in an unusually elongated and tilted ellipse.
It spends most of its orbital period beyond Neptune’s orbit, but is sometimes closer to the sun than Neptune.
Scientists think Pluto is made up of frozen methane, rock, and ice, with an average temperature of –235 °C. Pluto has extensive methane icecaps and a very thin nitrogen atmosphere.
Pluto has three moons, one of which, named Charon, has a diameter of more than half that of Pluto.

17. Objects Beyond Neptune, continued
In recent years, scientists have discovered hundreds of objects in our solar system beyond Neptune’s orbit. These objects are called trans-Neptunian objects (TNOs) and exist in the Kuiper Belt.
Kuiper Belt a region of the solar system that is just beyond the orbit of Neptune and that contains dwarf planets and other small bodies made mostly of ice
Eris, Makemake, and Haumea are trans-neptunian dwarf planets or plutoids, so called because they resemble Pluto. Other large TNOs have not been classified as such, but may eventually be considered as meeting the definition.

18. Reading check, continued
Where is the Kuiper Belt located?
The Kuiper Belt is located beyond the orbit of Neptune.

19. Exoplanets
Exoplanets are planetlike bodies that orbit stars other than Earth’s sun.
Exoplanets cannot yet be directly observed with telescopes or satellites. Most exoplanets can be detected only because their gravity tugs on stars that they orbit.
Most of the exoplanets that have been identified are larger than Uranus, but a few have been discovered that are closer to Earth in mass.

20. Light Year
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